We will investigate transplantable thyroid tumors of inbred Fischer rats following an approach that differs from present efforts in immunotherapy, which are mostly directed to the induction of a specific immune response, usually T lymphocyte-mediated, to tumor antigens, or to the stimulation of T cells by transfer factor or non-specific stimulants such as BCG, etc. We propose an alternate way, i.e., the induction of an autoimmune response to normal thyroid antigens and thus the stimulation of an inflammatory reaction that would involve not only normal thyroid tissue, but also the tumor itself. Sensitized lymphocytes in the thyroid should release a variety of lymphokines capable of affecting both normal and neoplastic tissue. From this point of view, even the presence of antibodies to thyroid antigens should not interfere with the destruction of tumor tissue, since it is highly unlikely that they would act as blocking factors. Their presence might actually be beneficial, increasing the inflammatory process through immune complex formation and/or antibody-dependent cell-mediated cytotoxicity. It is important to note that our approach is clinically feasible, since the thyroid is a "non-essential" organ, in that replacement therapy is available.